Compression mould for making a membrane electrode assembly

ABSTRACT

The invention provides a mold for use in a compression molding apparatus that has a frame part ( 41 ) with a hole through its center; a bottom plunger ( 42 ); and a top plunger ( 43 ); wherein the plungers are fabricated to fit substantially snugly in the hole in the frame part, and wherein at least one plunger comprises at least one low-thermal conductivity insert ( 44, 44 ′). The mold is useful in compression molding processes used in the preparation of unitized membrane electrodes.

FIELD OF THE INVENTION

This invention relates to a mold for use in compression molding, andmore particularly to the preparation of a unitized membrane electrodeassembly having fluid impermeable polymer seal that is prepared usingcompression molding.

BACKGROUND OF THE INVENTION

A variety of electrochemical cells falls within a category of cellsoften referred to as solid polymer electrolyte (“SPE”) cells. An SPEcell typically employs a membrane of a cation exchange polymer thatserves as a physical separator between the anode and cathode while alsoserving as an electrolyte. SPE cells can be operated as electrolyticcells for the production of electrochemical products or they may beoperated as fuel cells.

Fuel cells are electrochemical cells that convert reactants, namely fueland oxidant fluid streams, to generate electric power and reactionproducts. A broad range of reactants can be used in fuel cells and suchreactants may be delivered in gaseous or liquid streams. For example,the fuel stream may be substantially pure hydrogen gas, a gaseoushydrogen containing reformate stream, or an aqueous alcohol, for examplemethanol in a direct methanol fuel cell (DMFC). The oxidant may, forexample, be substantially pure oxygen or a dilute oxygen stream such asair.

In SPE fuel cells, the solid polymer electrolyte membrane is typicallyperfluorinated sulfonic acid polymer membrane in acid form.

Such fuel cells are often referred to as proton exchange membrane(“PEM”) fuel cells. The membrane is disposed between and in contact withthe anode and the cathode. Electrocatalysts in the anode and the cathodetypically induce the desired electrochemical reactions and may be, forexample, a metal black, an alloy or a metal catalyst supported on asubstrate, e.g., platinum on carbon. SPE fuel cells typically alsocomprise a porous, electrically conductive sheet material that is inelectrical contact with each of the electrodes, and permit diffusion ofthe reactants to the electrodes. In fuel cells that employ gaseousreactants, this porous, conductive sheet material is sometimes referredto as a gas diffusion backing and is suitably provided by a carbon fiberpaper or carbon cloth. An assembly including the membrane, anode andcathode, and gas diffusion backings for each electrode, is sometimesreferred to as a membrane electrode assembly (“MEA”). Bipolar plates,made of a conductive material and providing flow fields for thereactants, are placed between a number of adjacent MEAs. A number ofMEAs and bipolar plates are assembled in this manner to provide a fuelcell stack.

In fabricating unitized MEAs, multilayer MEAs may be sealed using afluid impermeable polymer seal. Several techniques may be used to formthese seals, including compression molding and injection molding. Withinjection molding, the sealing polymer that is used as the sealantmaterial is applied in liquid or slurry form and this is associated withits own disadvantages. In injection molding, the sealing polymersometimes does not flow onto both sides of the membrane, and therelatively high pressures and flow velocities may damage the gasdiffusion backings. Balancing the pressures on all edges of the gasdiffusion backings may be difficult. Another disadvantage of injectionmolding is the difficulty of maintaining the position of the componentsof the MEA in the mold. Clamping force on the components must be greatenough to impede motion due to the injection pressure and may damage thefibers in the gas diffusion backing, creating debris and possibleshorting of the MEA if the debris punctures the membrane. Sincecompression molding does not involve high-pressure gradients and flowvelocities, it does not generally have these problems.

A need exists for a mold useful in compression molding, whereinmembranes that are substantially dimensionally unstable are used, thatdoes not result in a damaged unitized MEA because of the application ofheat in the compression molding process.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a mold for use in compressionmolding comprising:

-   -   (a) a frame part with a hole through its center;    -   (b) a bottom plunger; and    -   (c) a top plunger; wherein the plungers are fabricated to fit        substantially snugly in the hole in the frame part, and wherein        at least one plunger comprises at least one low-thermal        conductivity insert.

In the first aspect, both plungers may be provided with at least onelow-thermal conductivity insert. Further, a plurality of plungers may beused instead of a single plunger.

In a second aspect, the invention provides a process of preparing aunitized membrane electrode assembly using compression-moldingcomprising:

-   -   (a) forming a multilayer sandwich comprising a first gas        diffusion backing having sealing edges; a first electrocatalyst        coating composition; a polymer membrane; a second        electrocatalyst coating composition; and a second gas diffusion        backing having sealing edges; and    -   (b) compression molding a sealing polymer to the multilayer        sandwich, wherein the mold used in the compression molding        process comprises:    -   (c) a frame part with a hole through its center;    -   (d) a bottom plunger; and    -   (e) a top plunger; wherein the plungers are fabricated to fit        substantially snugly in the hole in the frame part, and wherein        at least one plunger comprises at least one low-thermal        conductivity insert;        whereby the sealing polymer is impregnated into the sealing        edges of the first and second gas diffusion backings, and the        sealing polymer envelops a peripheral region of both the first        and second gas diffusion backings and the polymer membrane to        form a polymer, fluid impermeable seal. The sealing polymer may        be a thermosetting or curable resin polymer or a thermoplastic        polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a mold (40) used in compressionmolding showing the positioning of low-thermal conductivity inserts 44and 44′.

FIG. 2 is a schematic illustration of a expanded view of the mold (40).

FIG. 3 is a schematic illustration of plungers (42) or (43) showing morethan one insert in the same plunger.

FIG. 4 is a schematic illustration of a multilayer sandwich (10)comprising a first gas diffusion backing having sealing edges (13); afirst electrocatalyst coating composition (12); a polymer membrane (11);a second electrocatalyst coating composition (12′); and a second gasdiffusion backing having sealing edges (13′) used to form the membraneelectrode assembly (MEA)

FIG. 5 is a schematic illustration of a unitized MEA (30) after itsremoval from the mold in the compression molding process.

FIG. 6 is a schematic illustration of an oblique view of a unitized MEA(30) after its removal from the mold in the compression molding process.

DETAILED DESCRIPTION OF THE INVENTION

Compression Molding:

The compression-molding apparatus consists of a mold (40) and a heatedpress (not shown). The picture-frame mold (40) is fabricated of amaterial having high thermal conductivity and chosen to withstand theelevated temperatures of the process. Typically, a metal such as toolsteel or aluminum may be used. Some examples of metals that are usefulinclude metals having American Iron and Steel Institute (AISI)specifications of H-13, H-19, P-4, P-5, and P-6. Some additionalmaterials may include 400 series steels such as AISI 410, 416, 420, 431and 400. Some useful types of aluminum include Aluminum Association (AA)designations AA 5086, AA 5454, AA 2024, and AA 7075.

As shown in FIGS. 1 and 2, the mold (40) generally consists of threeparts—a frame part with a hole through its center (41), a bottom plunger(42), and a top plunger (43). The plungers are fabricated to fit snugglyinto the frame, and one of the plungers, typically the bottom plunger(42), may actually be integral with it. Typically, a hole (45) or (45′)is provided in the frame or plungers where a thermocouple may beinserted for the purpose of monitoring the sealing polymer temperature.

The plungers are typically to be heated and cooled at the same rate inorder to minimize warping of the product part. In one embodiment, thiscan be most readily achieved by making their masses essentially equal.Since the polymer membrane (11), such as an ion exchange membrane, thatis used to make the membrane electrode assembly is substantiallydimensionally unstable, it is important that the plungers be kept coolerin their centers than at their periphery. to prevent it from degradingor wrinkling. Plungers made with low-thermal conductivity inserts (44)and (44′) in their central areas can achieve this, as the center of themold may be insulated from the heat of the press, and therefore remainat a lower temperature than the metal parts throughout the process.Alternately, as shown in FIG. 3, the insert may be made up of aplurality of smaller inserts (44a′) with the proviso that the amount ofmetal kept in contact with the membrane electrode assembly issufficiently low so warping resulting from the metal contact isminimized. Any material may be used as the inserts (44) and (44′) withthe proviso that they keep the MEA components at a lower temperature.Some useful materials include ceramics selected from the groupconsisting of alumina, alumina silicate, glass, zirconia, and boronnitride. Some useful ceramic materials may be purchased from CotronicsCorporation, Brooklyn, N.Y., e.g. glass ceramics; Corning, Inc.,Corning, N.Y., e.g. ceramics sold under the tradename Macor®; MarylandLava Company, Street, Md., and Hottec, Inc., Norwich, Conn., e.g.cementous aluminate materials sold under the tradename Fabcram®.Adhesives may be used to bond the inserts in place. Some usefuladhesives in fast cure adhesives such as Zircon® adhesives, and theResbond™ family of adhesives from Cotronics Corporation, Brooklyn, N.Y.After bonding with the plunger the face of the plunger (46) is thenpolished so the insert is flush with the surface (46) of the plunger(42) or (43).

Membrane Electrode Assembly:

The unitized MEA is prepared using a multilayer sandwich (10), shown inFIG. 4, comprising a first gas diffusion backing having sealing edges(13); a first electrocatalyst coating composition (12); a polymermembrane (11); a second electrocatalyst coating composition (12′); and asecond gas diffusion backing having sealing edges (13′). The unitizedMEA also comprises a polymer fluid impermeable seal (14), shown in FIGS.5 and 6, wherein the sealing polymer is either a thermoplastic polymeror a thermosetting or curable resin, and wherein the sealing polymer isimpregnated into the at least a portion of the sealing edges of thefirst and second gas diffusion backings (13) and (13′), and the sealenvelops a peripheral region of both the first and second gas diffusionbackings (13) and (13′), and the polymer membrane (11).

Gas Diffusion Backing:

The gas diffusion backings having sealing edges (13) and (13′) include aporous electrically conductive material, typically having aninterconnected pore or void structure. Typically, the sealing edge ofthe gas diffusion backing is the cut edge. The electrically conductivematerial typically comprises a corrosion-resistant material such ascarbon, which may be formed into fibers. Such fibrous carbon structuresmay be in the form of a paper, woven fabric, or nonwoven web.Alternatively, the electrically conductive material may be in particleform. Mixtures of the fibrous carbon structures and the electricallyconductive material in particulate form may be used. The electricallyconductive material may further be optionally surface-treated to eitherincrease or decrease its surface energy, allowing it to have eitherincreased or decreased hydrophobicity.

A binder is optionally used to provide the structure with desiredmechanical properties such as strength or stiffness. The binder itselfmay be chosen to serve the additional purpose of a surface treatment asmentioned above.

A microporous composition may also be optionally included with one orboth of the gas diffusion backings. This composition may be located onone or both surfaces of the gas diffusion backing or impregnated into itor both. It serves, among other purposes, to afford electrical and/orfluid contact on a fine scale with the electrocatalyst coating. It mayfurther enhance the ability of the gas diffusion backing to permittwo-phase fluid flow during fuel cell operation, such as shedding liquidwater in the cathode oxidant stream or shedding carbon dioxide bubblesin the anode stream of a direct-methanol fuel cell. It typicallycomprises electrically conductive particles and a binder. The particlesmay be, for example, high-structure carbon black such as Vulcan® XC72manufactured by Cabot Corporation, or acetylene carbon black. The bindermay be, for example, a polymer such as Teflon® polytetrafluoroethylenemanufactured by DuPont.

First and Second Electrocatalyst Coating Compositions:

The electrocatalyst coating compositions (12) and (12′) include anelectrocatalyst and an ion exchange polymer; the two coatingcompositions may be the same or different. The ion exchange polymer mayperform several functions in the resulting electrode including servingas a binder for the electrocatalyst and improving ionic conductivity tocatalyst sites. Optionally, other components are included in thecomposition, e.g., PTFE in particle form.

Electrocatalysts in the composition are selected based on the particularintended application for the catalyst layer. Electrocatalysts suitablefor use in the present invention include one or more platinum groupmetal such as platinum, ruthenium, rhodium, and iridium andelectroconductive oxides thereof, and electroconductive reduced oxidesthereof. The catalyst may be supported or unsupported. For directmethanol fuel cells, a (Pt—Ru)O_(X) electocatalyst has been found to beuseful. One particularly preferred catalyst composition for hydrogenfuel cells is platinum on carbon, for example, 60-wt % carbon, 40-wt %platinum, obtainable from E-Tek Corporation of Natick, Mass. Thesecompositions when employed accordance with the procedures describedherein, provided particles in the electrode which are less than 1 μm insize.

Since the ion exchange polymer employed in the electrocatalyst coatingcomposition serves not only as binder for the electrocatalyst particlesbut also may assist in securing the electrode to the membrane, it ispreferable for the ion exchange polymers in the composition to becompatible with the ion exchange polymer in the membrane. Mostpreferably, exchange polymers in the composition are the same type asthe ion exchange polymer in the membrane.

Ion exchange polymers for use in accordance with the present inventionare preferably highly fluorinated ion-exchange polymers. “Highlyfluorinated” means that at least 90% of the total number of univalentatoms in the polymer are fluorine atoms. Most preferably, the polymer isperfluorinated. It is also preferred for use in fuel cells for thepolymers to have sulfonate ion exchange groups. The term “sulfonate ionexchange groups” is intended to refer to either sulfonic acid groups orsalts of sulfonic acid groups, preferably alkali metal or ammoniumsalts. For applications where the polymer is to be used for protonexchange as in fuel cells, the sulfonic acid form of the polymer ispreferred. If the polymer in the electrocatalyst coating composition isnot in sulfonic acid form when used, a post treatment acid exchange stepwill be required to convert the polymer to acid form prior to use.

Preferably, the ion exchange polymer employed comprises a polymerbackbone with recurring side chains attached to the backbone with theside chains carrying the ion exchange groups. Possible polymers includehomopolymers or copolymers of two or more monomers. Copolymers aretypically formed from one monomer which is a nonfunctional monomer andwhich provides carbon atoms for the polymer backbone. A second monomerprovides both carbon atoms for the polymer backbone and also contributesthe side chain carrying the cation exchange group or its precursor,e.g., a sulfonyl halide group such a sulfonyl fluoride (—SO₂F), whichcan be subsequently hydrolyzed to a sulfonate ion exchange group. Forexample, copolymers of a first fluorinated vinyl monomer together with asecond fluorinated vinyl monomer having a sulfonyl fluoride group(—SO₂F) can be used. Possible first monomers include tetrafluoroethylene(TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride,trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkyl vinylether), and mixtures thereof. Possible second monomers include a varietyof fluorinated vinyl ethers with sulfonate ion exchange groups orprecursor groups which can provide the desired side chain in thepolymer. The first monomer may also have a side chain that does notinterfere with the ion exchange function of the sulfonate ion exchangegroup. Additional monomers can also be incorporated into these polymersif desired.

Especially preferred polymers for use in the present invention include ahighly fluorinated, most preferably perfluorinated, carbon backbone witha side chain represented by the formula—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₃H, wherein R_(f) and R′_(f) areindependently selected from F, Cl or a perfluorinated alkyl group having1 to 10 carbon atoms, a=0, 1 or 2. The preferred polymers include, forexample, polymers disclosed in U.S. Pat. No. 3,282,875 and in U.S. Pat.Nos. 4,358,545 and 4,940,525.

The electrocatalyst coating or catalyst layer may be formed from aslurry or ink. The liquid medium for the ink is one selected to becompatible with the process of application. The inks may be applied tothe membrane by any known technique to form a catalyst-coated membrane.Alternately, the inks may be applied to the gas diffusion backing. Someknown application techniques include screen, offset, gravure,flexographic or pad printing, or slot-die, doctor-blade, dip, or spraycoating. It is advantageous for the medium to have a sufficiently lowboiling point that rapid drying of electrode layers is possible underthe process conditions employed. When using flexographic or pad printingtechniques, it is important that the composition not dry so fast that itdries on the flexographic plate or the cliche plate or the pad beforetransfer to the membrane film.

A wide variety of polar organic liquids or mixtures thereof can serve assuitable liquid media for the ink. Water in minor quantity may bepresent in the medium if it does not interfere with the printingprocess. Some preferred polar organic liquids have the capability toswell the membrane in large quantity although the amount of liquids theelectrocatalyst coating composition applied in accordance with theinvention is sufficiently limited that the adverse effects from swellingduring the process are minor or undetectable. It is believed thatsolvents with the capability to swell the polymer membrane can providebetter contact and more secure application of the electrode to themembrane. A variety of alcohols are well suited for use as the liquidmedium.

Preferred liquid media include suitable C4 to C8 alkyl alcoholsincluding, n-, iso-, sec- and tert-butyl alcohols; the isomeric 5-carbonalcohols, 1,2- and 3-pentanol, 2-methyl-1-butanol, 3-methyl, 1-butanol,etc., the isomeric 6-carbon alcohols, e.g. 1-, 2-, and 3-hexanol,2-methyl-1-pentanol, 3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl,1-pentanol, 4-methyl-1-pentanol, etc., the isomeric C7 alcohols and theisomeric C8 alcohols. Cyclic alcohols are also suitable. Preferredalcohols are n-butanol and n-hexanol. Most preferred is n-hexanol.

If the polymer in the electrocatalyst coating composition is not insulfonic acid form when used, a different liquid medium may be preferredin the ink. For example, if the one of the preferred polymers describedabove has its sulfonated groups in the form of sulfonyl fluoride, apreferred liquid medium is a high-boiling fluorocarbon such as“Fluorinert” FC-40 manufactured by 3M.

Handling properties of the ink, e.g. drying performance, can be modifiedby the inclusion of compatible additives such as ethylene glycol orglycerin up to 25% by weight based on the total weight of liquid medium.

It has been found that the commercially available dispersion of the acidform of the perfluorinated sulfonic acid polymer, sold by E.I. du Pontde Nemours and Company under the trademark Nafion®, in a water/alcoholdispersion, can be used, as starting material, for the preparation of anelectrocatalyst coating composition suitable for use in flexographic orpad printing.

In the electrocatalyst coating composition, it is preferable to adjustthe amounts of electrocatalyst, ion exchange polymer and othercomponents, if present, so that the electrocatalyst is the majorcomponent by weight of the resulting electrode. Most preferably, theweight ratio of electrocatalyst to ion exchange polymer in the electrodeis about 2:1 to about 10:1.

Utilization of the electrocatalyst coating technique in accordance withthe process of the present invention can produce a wide variety ofprinted layers which can be of essentially any thickness ranging fromvery thick, e.g., 20 μm or more very thin, e.g., 1 μm or less. This fullrange of thickness can be produced without evidence of cracking, loss ofadhesion, or other inhomogenieties. Thick layers, or complicatedmulti-layer structures, can be easily achieved by utilizing the patternregistration available using flexographic or pad printing technology toprovide multiple layers deposited onto the same area so that the desiredultimate thickness can be obtained. On the other hand, only a few layersor perhaps a single layer can be used to produce very thin electrodes.Typically, a thin layer ranging from 1 to 2 μm may be produced with eachprinting with lower % solids formulations. Some typical electrostaticcoating compositions or inks are disclosed in U.S. Pat. No. 5,330,860.

The multilayer structures mentioned above permit the electrocatalystcoating to vary in composition, for example the concentration ofprecious metal catalyst can vary with the distance from the substrate,e.g. membrane, surface. In addition, hydrophilicity can be made tochange as a function of coating thickness, e.g., layers with varying ionexchange polymer EW can be employed. Also, protective orabrasion-resistant top layers may be applied in the final layerapplications of the electrocatalyst coating.

Composition may also be varied over the length and width of theelectrocatalyst coated area by controlling the amount applied as afunction of the distance from the center of the application area as wellas by changes in coating applied per pass. This control is useful fordealing with the discontinuities that occur at the edges and corners ofthe fuel cell, where activity goes abruptly to zero. By varying coatingcomposition or plate image characteristics, the transition to zeroactivity can be made gradual. In addition, in liquid feed fuel cells,concentration variations from the inlet to the outlet ports can becompensated for by varying the electrocatalyst coating across the lengthand width of the membrane.

Polymer Membrane:

A polymer membrane (11), for use in accordance with the invention, canbe made of the same ion exchange polymers discussed above for use in theelectrocatalyst coating compositions. The membranes can be made by knownextrusion or casting techniques and have thickness which can varydepending upon the application and typically have a thickness of about350 μm or less. The trend is to employ membranes that are quite thin,i.e., about 50 μm or less. The process in accordance with the present ininvention is well-suited for use in forming electrodes on such thinmembranes where the problem associated with large quantities of solventduring coating are especially pronounced. While the polymer may be inalkali metal or ammonium salt form during the flexographic or padprinting process, it is preferred for the polymer in the membrane to bein acid form to avoid post treatment acid exchange steps. Suitableperfluorinated sulfonic acid polymer membranes in acid form areavailable under the trademark Nafion® by E.I. du Pont de Nemours andCompany. Alternatively, membranes made from a variety of otherion-conducting polymers could be used, for example sulfonatedpolyaromatics as described in World Patent WO 00/15691.

Reinforced perfluorinated ion exchange polymer membranes can also beutilized in catalyst coated membrane (CCM) manufacture by the inventiveprinting process. Reinforced membranes can be made by impregnatingporous, expanded PTFE (ePTFE) with ion exchange polymer. Expanded PTFEis available under the tradename “Goretex” from W. L. Gore andAssociates, Inc., Elkton Md., and under the tradename “Tetratex” fromTetratec, Feasterville Pa. Impregnation of ePTFE with perfluorinatedsulfonic acid polymer is disclosed in U.S. Pat. Nos. 5,547,551 and6,110,333.

Catalyst coated membranes or gas diffusion backings coated withelectrocatalyst coating compositions may be provided with posttreatments such as calendering, vapor treatment to affect watertransport, or liquid extraction to remove trace residuals from any ofthe above earlier steps. If the membrane dispersion or solution used wasthe precursor of the highly fluorinated ionomer, after application ofthe solution or dispersion the sandwich formed may be subjected to achemical treatment to convert the precursor to the ionomer.

Sealing Polymer:

Thermosetting or thermoplastic polymers may be used as the sealingpolymer. FIG. 5 is a schematic illustration of a unitized MEA (30) afterits removal from the mold in the compression molding process.

Thermoplastic polymers are “materials that soften and flow uponapplication of pressure and heat. Thus, most thermoplastic materials canbe remolded many times. The obvious advantage is that a piece that isrejected or broken after molding can be ground up and remolded. In caseof a mis-molded part, thermoplastic materials also offer the option ofrepair through application of heat. Some techniques for this include,for example, contact heating, infrared energy, and ultrasonic welding.The presence of electrical conductors in a fuel cell also offers thepossibility of electrical resistance or induction welding to re-melt andre-form a thermoplastic component.

Chemically, thermoplastic processing is essentially inert, with very lowemissions and little or no appreciable chemical reaction-taking place.Thus, problems such as environmental impact, worker exposure, and bubbleformation in the parts are minimal. Thermoplastics as a class includesome of the most chemically inert materials in common usage, such asfluoropolymers and aromatic poly(ether ketone)s. Such sealing polymersare available with extremely low levels of any potential fuel cellcontaminants, such as metals, catalysts, and reactive functional groups.

Thermoplastic polymers offer a wide range of physical properties ofinterest to the fuel cell designer. Semicrystalline forms such ashigh-density polyethylene and polyvinylidene fluoride have particularlylow permeability to gases and liquids, and high mechanical toughness.Many have high compressive moduli, either in the neat or reinforcedforms, and so can be used to rigidly support fuel cell stack pressurewithout significantly changing the MEA thickness. Finally,thermoplastics such as melt-processible fluoropolymers offer verydurable electrical properties, including dielectric strength andelectrical resistance.

One of the most significant advantages for thermoplastics in thisapplication is their flow properties. In the process of injectionmolding, the mold and MEA are held below the melt temperature of theinjected sealing polymer as it is introduced. The sealing polymersolidifies almost instantly upon contact with these relatively coolsurfaces, and additional sealing polymer continues to flow past theseareas in the interior of the cavity only. As this material reaches theflow front, it spreads apart, contacts cooler surface, and solidifiesthere. This phenomenon, referred to as “fountain flow” inpolymer-processing literature, offers a unique advantage forthermoplastics in this invention. The spreading-apart effect tends toseparate electrodes that were initially in or near short-circuitcontact. Further, the rapid solidification on contact with the MEAlayers tends to prevent sealing off of the catalyst layers. Theelectrode-separating action of thermoplastic flow has been clearly seenin the products of this invention through microscopic examination.

The thermoplastic polymers useable in this invention may be from any ofa number of classes. Melt-processible fluoropolymers such as DuPontTeflon® FEP 100 and DuPont Teflon® PFA 340 may be used, as well aspartially fluorinated polymers, an example being polyvinylidene fluoridesuch as Kynar® 710 and Kynar Flex® 2801 manufactured by AtofinaChemicals, King of Prussia, Pa. Thermoplastic fluoroelastomers such asKalrez® and Viton®, manufactured by E. I. Du Pont de Nemours & Company,Inc., Wilmington, Del., also fall into this class. Aromatic condensationpolymers such as polyaryl(ether ketone)'s, an example beingpolyaryl(ether ether ketone) manufactured by Victrex ManufacturingLimited, Lancashire, Great Britain; modified polyethylene such as Bynel®40E529, modified polypropylene such as Bynel® 50E561, both manufacturedby DuPont; polyethylene such as Sclair® 2318 manufactured by NOVAChemicals Corporation, Calgary, Alberta, Canada; thermoplasticelastomers such as Hytrel® (DuPont); liquid-crystal polymers such asZenite® liquid-crystal polyester (DuPont), and aromatic polyamides suchas Zytel® HTN (DuPont) can also be used. Thermosetting materials arematerials that, once heated, react irreversibly so that subsequentapplications of heat and pressure do not cause them to soften and flow.Thermoplastic polymers are preferred over thermosetting materialsbecause a rejected or scrapped piece prepared with a thermosettingmaterial cannot be ground up and remolded. Some examples ofthermosetting materials include epoxies, urethane resins, and vulcanizednatural rubber.

The sealing polymer may also be optionally reinforced with fibers,fabrics, or inorganic fillers, which may either be placed in the moldduring the compression molding process or compounded into the sealingpolymer beforehand. Such reinforcements can reduce warpage in the finalpart.

Process:

The multilayer MEA sandwich (10) is placed in the center of the bottomplunger (42) with the frame part (41) around it. The plunger may have arelease surface or be optionally coated or lined with a release agent,such as PTFE film, to allow easy removal of the part after molding. TheCCM or membrane is typically cut to be larger than the gas diffusionbackings. Several layers of the sealing polymer film are cut to theshape of a frame to surround the gas diffusion backings (13) and (13′)but partly overlap the extended portion of the membrane (11) all aroundits perimeter. Alternatively, the sealing polymer may be introduced tothe process in a number of other forms, including powders, strips,fibers, fabric, liquid, or paste. It is preferable that it be introducedin a precisely metered manner, such as a die-cut film of controlledthickness or a metering pump with robotic control for a liquid. Thesealing polymer is placed in the mold, above and below the membrane butsurrounding the gas diffusion backings. As with the bottom plunger (42),the top plunger (43) may have a release surface or be optionally coatedor lined with a release agent, such as PTFE film.

The top plunger (43) of the tool is put in place, fitted into the framepart (41). The tool (40) with the materials within is put in a press,allowed to heat to above the melting point of the sealing polymer,compressed by mechanical action of the press, for example hydraulically,and cooled in place. The press may be heated on only one side in whichcase on the plunger on the side that is heated needs a low-thermalconductivity insert. Any press suitable for heating and melting thethermoplastic seal material may be used in this invention. Some knownpresses include presses from Carver Inc., Wabash, Ind.; PHI, City OfIndustry, Calif.; and Johnson Machinery Company, Bloomfield, N.J. A shim(not shown) may be placed on the frame between the top plunger and theframe to determine the extent to which the MEA components arecompressed. If a shim is not used a compression pressure of about 0.1 toabout 10 MPa, more typically a compression pressure of about 2 to about3 MPa may be used. The sealing polymer is preferably heated to just thepoint of complete melting throughout before cooling is initiated. Afterthe sealing polymer is cooled sufficiently for it to have structuralintegrity, the unitized MEA, shown in FIGS. 5 and 6 was removed from thetool. As can be clearly seen the unitized MEA (30) comprises the MEAsandwich (11) and an integral seal (14). The unitized MEA may also becooled to lower temperatures if necessary, for example to reducewarpage.

Ridges, ribs and other features (not shown) may be provided on the sealby having recesses in the plunger area adjacent the seal.

An example of a well-known industrial process of compression molding wasthe production of phonograph records, which were typically made fromcompounded polyvinyl chloride. An example of such a process is describedin Principles of Polymer Systems, 2nd Ed., Ferdinand Rodriguez,McGraw-Hill, New York, 1982.

Fuel Cell:

The unitized MEA (30) may be used to assemble a fuel cell. Bipolarplates (not shown) are positioned on the outer surfaces of the first andsecond (cathode and anode) gas diffusion backings having sealing edges(13) and (13′). If the seals (14) and (14′) are provided with ridges,domes, ribs, or other structural features (not shown), the bipolarplates may be provided with recesses that mesh with these features onthe seals (14) and (14′).

Several fuel cells may be connected together, typically in series, toincrease the overall voltage of the assembly. This assembly is typicallyknown as a fuel cell stack.

Use of the mold having a low conductivity insert and manufacture of theunitized MEA of the invention will be further clarified with referenceto the following examples. The examples are merely illustrative and arenot intended to limit the scope of the invention.

EXAMPLES

Control 1:

A picture-frame mold was fabricated of tool steel, having an AmericanIron and Steel Institute (AISI) specification of H-13 heat treated to RC40-44, and manufactured by Carpenter Technology Corporation, Reading,Pa. The mold consisted of three parts—a frame with 7.6-cm-square hole, a0.95-cm-thick 7.6-cm-square bottom plunger, and a 4.1-cm-thick,7.6-cm-diameter square top plunger. A hole was drilled into one side ofthe frame where a thermocouple was inserted for the purpose of readingthe mold temperature at this interface. Steel shims having an equalthickness are placed on opposite sides of the frame between the frameand the top plunger to limit the amount of compression in the MEA.

A three-layer sandwich comprising 0.2-mm-thick Nafion® 117, DuPont,Wilmington, Del., between two layers of a carbon-fiber-based diffusionbacking, SGL “Sigracet” GDL 10AA, manufactured by SGL Carbon Group,Manheim, Germany, was placed in the center of the bottom plunger atop a0.08-mm-thick PTFE release film, with the frame part around it. Thissandwich was in essence a “dummy” MEA, in that it lacked theelectrocatalysts necessary for fuel cell function, but could servemechanically and electrically to work in the same way.

The membrane had been cut to be about 7 mm larger than the diffusionbackings. Several layers of thermoplastic polymer film, Bynel® 40E529polyethylene-containing seal material, manufactured by DuPont,Wilmington, Del., were die-cut to square dimensions of 7.6 cm outsidediameter and 5.1 cm inside diameter; the films thus formed frames thatwould surround the diffusion backings but partly overlap the extendedportion of the membrane all around its perimeter. These layers ofsealing polymer were also placed in the mold, above and below themembrane. A second piece of the release film was placed on top of thesandwich.

The top plunger of the tool was put in place, fitted into the frame. Thetool with the materials within was placed in a press, allowed to heat toabove the melting point of the thermoplastic polymer, compressedhydraulically and cooled in place. Just before cooling, the temperaturein the frame was measured to be approximately 185° C., and the set pointfor the press-platens temperature was 200° C. After the frametemperature was below 60° C., the part was removed from the tool.

The sandwich components were held together by the consolidatedthermoplastic polymer seal thus formed. Further, the MEA sandwich wasfully encapsulated at its edges; the edge of the membrane was notvisible around any of the specimen. The sealing polymer was able tocontact and slightly ingress into both of the diffusion backing layersall around their perimeters. However, the product specimen wassignificantly warped, and the central part of the membrane appearedrippled and was found to be dark in color, indicating it had beenoverheated.

Example 1

Control 1 was repeated with the following exception: the top and bottomplungers of the tool were modified so only the areas contacting theBynel® 40E529 polyethylene-containing seal material, manufactured byDuPont, Wilmington, Del., were steel. The inner square area adjacent thegas diffusion backings, were made of a low-thermal-conductivity ceramicCotronics 914 machinable glass ceramic, manufactured CotronicsCorporation, Brooklyn, N.Y., and the inner surface of this ceramic wasmaintained at a much lower temperature than the steel portion throughoutthe molding process. Further, the molding temperature was reduced; theset point temperature was maintained at 145° C. and the maximum frametemperature was maintained at 137° C.

The specimen thus made was fully encapsulated, and the central part ofthe membrane was smooth and flat, showing no ripples. However, thespecimen was still somewhat warped indicating that depending on thecomponents of the sandwich the mass of the top and bottom plungers mayhave to adjusted to avoid warping of the unitized MEA formed.

Example 2

Example 1 was repeated with the following exception: the tooling wasmodified such that the top and bottom plungers of the tool were the samethickness, 4.1 cm, and mass. The frame was suspended high enough, usingshims, such that its midpoint was near the midpoint of the MEAmaterials.

The specimen thus made was fully encapsulated, and was not warped, andthe central part of the membrane was smooth and flat, showing noripples.

Example 3

Example 2 was repeated with the following exception: the bottom plungerwas replaced with an unmodified plunger containing no ceramic material.

The specimen thus made was fully encapsulated, but exhibited minorwarping, and the central part of the membrane was smooth and flat,showing no ripples.

Example 4

Example 2 was repeated with the following exception: instead of a0.2-mm-thick Nafion® membrane, a 0.05-mm-thick membrane coated on bothsides with a platinum-based catalyst layer was used. Thiscatalyst-coated membrane (CCM) was designed for use in a PEM fuel cell.

The specimen thus made was placed in a hydrogen-fueled test fuel celland found to generate electric current. A polarization curve wasgenerated for this specimen and found to match that of a similar CCMassembled into a similar cell with traditional gaskets and separatediffusion backings.

1. A mold for use in a compression molding apparatus comprising: (a) aframe part with a hole through its center; (b) a bottom plunger; and (c)a top plunger; wherein the plungers are fabricated to fit substantiallysnugly in the hole in the frame part, and wherein at least one plungercomprises at least one low-thermal conductivity insert.
 2. The mold ofclaim 1 wherein the at least one low-thermal conductivity insert ispresent in the top plunger and the bottom plunger.
 3. The mold of claim1 wherein the plunger comprises a plurality of low-thermal conductivityinserts.
 4. The mold of claim 3 wherein the at least low-thermalconductivity insert is a ceramic.
 5. The mold of claim 4 wherein theceramic is selected from the group consisting of alumina, aluminasilicate, glass, zirconia, and boron nitride.
 6. The mold of claim 1wherein the at least one low-thermal conductivity insert is bonded tothe at least one plunger with an adhesive.
 7. The mold of claim 1wherein the adhesive is a fast cure adhesive.
 8. A process of preparinga unitized membrane electrode assembly using compression moldingcomprising: (a) forming a multilayer sandwich comprising a first gasdiffusion backing having sealing edges; a first electrocatalyst coatingcomposition; a polymer membrane; a second electrocatalyst coatingcomposition; and a second gas diffusion backing having sealing edges;and (b) compression molding a sealing polymer to the multilayersandwich, wherein the mold used in the compression molding apparatuscomprises: (c) a frame part with a hole through its center; (d) a bottomplunger; and (e) a top plunger; wherein the plungers are fabricated tofit substantially snuggly in the hole in the frame part, and wherein atleast one plunger comprises at least one low-thermal conductivityinsert; whereby the sealing polymer is impregnated into at least aportion of the sealing edges of the first and second gas diffusionbackings, and the thermoplastic polymer envelops a peripheral region ofboth the first and second gas diffusion backings and the polymermembrane to form a thermoplastic polymer, fluid impermeable seal.
 9. Theprocess of claim 8 wherein the at least one low-thermal conductivityinsert is present in the top plunger and the bottom plunger.
 10. Theprocess of claim 8 wherein the plunger comprises a plurality oflow-thermal conductivity inserts.
 11. The process of claim 10 whereinthe at least one low-thermal conductivity insert is a ceramic.
 12. Theprocess of claim 11 wherein the ceramic is selected from the groupconsisting of alumina, alumina silicate, glass, zirconia, and boronnitride.
 13. The process of claim 8 wherein the at least one low-thermalconductivity insert is bonded to the at least one plunger with anadhesive.
 14. The process of claim 13 wherein the adhesive is a fastcure adhesive.
 15. The process of claim 8 wherein the first and secondelectrocatalyst coating composition are coated on opposite sides of thepolymer membrane to form a catalyst-coated membrane.
 16. The process ofclaim 8 wherein membrane and the first and second gas diffusion backingshaving sealing edges are all the same size.
 17. The process of claim 15wherein the catalyst coated membrane and the first and second gasdiffusion backings having sealing edges are all the same size.
 18. Theprocess of claim 8 wherein the membrane is longer than the first andsecond gas diffusion backings having sealing edges.
 19. The process ofclaim 15 wherein the catalyst coated membrane is longer than the firstand second gas diffusion backings having sealing edges.
 20. The processof claim 15 wherein the sealing polymer comprises a thermoplasticpolymer.
 21. The process of claim 15 wherein the sealing polymercomprises a thermosetting or curable-resin polymer.